It is known in the art that the velocity of a sound wave in a material depends on the mechanical properties of the material. This is outlined by C. H. Hastings and S. W. Carter in an article entitled "Inspection, Processing and Manufacturing Control of Metal by Ultrasonic Methods", Symposium on Ultrasonic Testing, 52 nd Annual Meeting of the American Society for Testing Materials, Jun. 28, 1949, pp. 16-47.
U.S. Pat. Nos. 3,720,098, 3,228,232, 3,288,241, 3,372,163, 3,127,950, 3,512,400, 4,640,132, 4,597,292 and 4,752,917 describe the state of the art of non-destructive testing.
A sound wave which reaches a semi-infinite solid at an angle will typically propagate through the solid as three waves, namely, the longitudinal, transverse and surface waves, wherein each wave has a different velocity. As described by Hastings and Carter, the velocities of the three waves are defined as follows: ##EQU1## where V.sub.L, V.sub.T, and V.sub.S are, respectively, the velocities of the longitudinal, transverse and Rayleigh surface waves, and E, .sigma. and rho are, respectively, the Young's Modulus, the Poisson's ratio of lateral contraction to longitudinal extension and the mass density of the material. Equation 3b is an empirical relationship as defined on page 326 of Wave Motion in Elastic Solids, by Karl F. Graff, published by the Clarendon Press, Oxford England in 1975.
In ultrasonic measurement of the condition of bone, typically only the velocity of the longitudinal wave is used. As defined in the article, "Osteoporotic Bone Fragility: Detection by Ultrasound Transmission Velocity," R. P. Heaney et al,. JAMA, Vol. 261, No. 20, May 26, 1989, pp. 2986-2990, the Young's modulus of a bone E is given empirically as: EQU E=K (rho).sup.2 ( 4a)
and the velocity of sound through the bone is a function of E, where the velocity of :sound is typically the longitudinal velocity, as follows: EQU V.sub.L =.sqroot.(E/rho)=.sqroot.(K*rho) (4b)
where K is a constant which incorporates a number of factors, such as spatial orientation of the bone structures, inherent properties of the bone material and fatigue damage. Thus, the velocity of a longitudinal wave is a function of the mass density and can be used as an indicator of the quality of bone.
The following articles also discuss ultrasonic measurement of bone condition both in vivo and in vitro.
"Measurement of the Velocity of Ultrasound in Human Cortical Bone In Vivo," M. A. Greenfield, et al., Radiology, Vol. 138, March 1981, pp. 701-710.
"Combined 2.25 MHz ultrasound velocity and bone mineral density measurements in the equine metacarpus and their in vivo applications," R. N. McCartney and L. B. Jeffcott, Medical and Biological Engineering and Computation, Vol. 25, 1987, Nov. 1877, pp. 620-626.
In order to perform in vivo ultrasonically measurements of the mechanical properties of bone, it is necessary to transmit an ultrasonic wave through the soft tissue surrounding the bone. Unfortunately, the thickness of the soft tissue varies along the length of the bone. This can affect the accuracy of the ultrasound propagation time measurement through the bone. In the abovementioned articles, the thickness of the soft tissue is either ignored or an attempt is made to cancel the effects of the soft tissue. In the articles describing in vitro experiments, the soft tissue is removed from the bone.
Russian patents 1,420,383, 1,308,319, 1,175,435, 1,324,479, 1,159,556 and 1,172,534 and U.S. Pat. Nos. 4,926,870, 4,361,154, 4,774,959, 4,421,119, 4,941,474, 3,847,141, 4,913,157 and 4,930,511 describe various systems for measuring the strength of bone based on the velocity V.sub.L. These systems typically have one ultrasonic signal transmitter and at least one ultrasonic signal receiver.
Russian patents 1,420,383, 1,308,319 and 1,175,435 solve the problem of the unknown thickness of the soft tissue by assuming values for the thickness of the soft tissue in the area of the measurement or by assuring that the thickness variation is small over the distance between two ultrasonic signal receivers.
Russian patent 1,342,279 utilizes two receivers and a single transmitter and calculates an average group speed through the bone based on the known distance between the two receivers.
Russian patent 1,159,556 defines zones of a bone and the condition of a bone is determined by the difference between the maximum and minimum amplitude of the ultrasound signals measured. It would appear that this measurement is performed on an excised bone.
Russian patent 1,172,534 describes a system which compares the ultrasound signal of a healthy bone with that of an unhealthy bone and from the comparison, produces a diagnosis of the extent of disease in the unhealthy bone.
U.S. Pat. Nos. 4,926,870, 4,421,119 and 3,847,141 describe systems which places a receiver and a transmitter on opposite sides of a bone. U.S. Pat. No. 4,926,870 also compares the resultant signal with a canonical waveform, thereby to identify the health of the bone.
U.S. Pat. Nos. 4,913,157, 4,774,959 and 4,941,474 describe systems which transmit an ultrasonic signal with a spectrum of frequencies.
U.S. Pat. No. 4,930,511 describes a system is placed around a standard inanimate homogeneous material of known acoustic properties before it is placed around a bone.